There is a wide choice of host organisms, such as modified mammal cells or microorganisms, which can potentially be used for the purpose of producing large quantities of human proteins of high therapeutic value.
The use of modified mammal cells with recombinant DNA techniques has the advantage of resulting in products which are closely related to those of natural origin; however, the culturing of these cells is intricate and can only be carried out on a limited scale.
The use of microorganisms such as bacteria permits manufacture on a larger scale, but introduces the disadvantage of producing products which differ appreciably from the products of natural origin. Thus, the proteins which are usually glycosylated in man are, in general, not glycosylated by bacteria [P. Berman and L. A. Laskey, Trends Biochem. Sci., (1985), 10, p.51 et seq]. Furthermore, human proteins which are expressed at a high level in bacteria such as E. coli frequently acquire an unnatural conformation which is accompanied by an intracellular precipitation [R. G. Schoner et coll., Bio. Technol. (1985), 3, p.151 et seq; J. M. Schoemaker et coll., EMBO J. (1985), 4, p.775 et seq]. Lastly, to enable a gene to be expressed in a bacterium, such as E. coli, it is essential that a methionine initiator codon is positioned before the coding sequence for the natural protein. In general, this residue is not excised by the methionyl aminopeptidase of E. coli [P. H. Seeburg et coll., 1985, 2, p.37 et seq; J. M. Schoner et coll., Proc. Natl. Acad. Sci. USA (1981), 81, p.5403].
The protein obtained thus has an abnormal amino acid as first residue, which can give rise to steric inhibition of biological activity when the beginning of the protein is involved in the activity. The residue may also be of an immunogenic character which is detrimental to the subsequent administration of the protein.